The "may not" portion of the message is mostly referring to advanced features of higher versions (so the Voronoi Fracture from R18 cannot be used when loaded into R15, for example). The file format is basically fully compatible. So, if you press "OK", the file will be saved for R18, but as long as you do not add advanced features, it will remain backwards compatible.

Caustics require a big enough light to be correctly sampled. The HDRI or the Sun/Sky Object won't help much. Also, the PPM engine will fuck many of your other materials. Try using a huge circular Area light directed at the pool with directionality ~60.It's best to only have the top part of the mesh be displaced and not ripped off from the rest of the box. Also, a portal might help.